Chlorine is often present in water from municipal sources as a result of disinfection processes. Chlorine (including chloramine) levels in municipal water supplies range up to about 3 ppm. These chlorines and chloramines, together with other substances which may be dissolved or present in municipal water supplies, affect its taste. They also control the water's pH at point of use which, when supplied from municipal water sources, is usually within a range of 6 to 9 pH and under EPA standards should generally be within a range of 6.5 to 8.5 pH.
Activated carbon filters at point of use comprise a well known means for removing chlorine from water. However, there are significant expenses incident to modification of plumbing, as well as the provision for and replacement of the filters. Moreover, a pressure drop occurs across the filters and their effectiveness for the removal of chlorines and chloramines is not always dependable.
It is also known that salts of the sulphur oxide family can be useful dehalogenation agents. Such agents include iron sulfate and thiosulfates such as sodium thiosulfate.
When adding sulfur oxides such as, for example, sodium sulfite and thiosulfates, to water, it is considered important to keep their proportions rather closely to those required for the reaction with the chlorine and chloramines therein, because greater proportions can cause a reducing effect which, although not injurious to health, is undesirable, and lesser amounts are not effective. When a thiosulfate is used to destroy the chlorine and chloramines resulting from hypochlorite disinfectants added to the water, these undesired substances are destroyed by an autocatalytic reaction with the thiosulfate at a molar ratio of about 0.25 moles of thiosulfate to 1 mole of hypochlorite.
Sulfite reaction times with chlorines and chloramines in tap water are reported to require minutes and sometimes hours or even days, and to be substantially dependent upon the temperature of the solutions, their pH values, and the particle sizes of the sulfites.
Sodium thiosulfate, which is the most readily available of the thiosulfates, is also known to have a cooling taste with a bitter aftertaste. It is, however, generally considered harmless and, for some purposes, beneficial in doses of a gram or less, and the same is true for calcium thiosulfate and magnesium thiosulfate.